linux-kernel - [PATCH 3/4] hfsplus: Convert kmap() to kmap_local

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Message-Id: <20220809203105.26183-4-fmdefrancesco@gmail.com>
Date:   Tue,  9 Aug 2022 22:31:04 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>, Jens Axboe <axboe@...nel.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bart Van Assche <bvanassche@....org>,
        Kees Cook <keescook@...omium.org>,
        Muchun Song <songmuchun@...edance.com>,
        Viacheslav Dubeyko <slava@...eyko.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 3/4] hfsplus: Convert kmap() to kmap_local_page() in bitmap.c

kmap() is being deprecated in favor of kmap_local_page().

There are two main problems with kmap(): (1) It comes with an overhead as
mapping space is restricted and protected by a global lock for
synchronization and (2) it also requires global TLB invalidation when the
kmap’s pool wraps and it might block when the mapping space is fully
utilized until a slot becomes available.

With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts).
It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
the tasks can be preempted and, when they are scheduled to run again, the
kernel virtual addresses are restored and are still valid.

Since its use in bitmap.c is safe everywhere, it should be preferred.

Therefore, replace kmap() with kmap_local_page() in bitmap.c.

Tested in a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
HIGHMEM64GB enabled.

Cc: Viacheslav Dubeyko <slava@...eyko.com>
Suggested-by: Ira Weiny <ira.weiny@...el.com>
Reviewed-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---
 fs/hfsplus/bitmap.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/fs/hfsplus/bitmap.c b/fs/hfsplus/bitmap.c
index cebce0cfe340..bd8dcea85588 100644
--- a/fs/hfsplus/bitmap.c
+++ b/fs/hfsplus/bitmap.c
@@ -39,7 +39,7 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 		start = size;
 		goto out;
 	}
-	pptr = kmap(page);
+	pptr = kmap_local_page(page);
 	curr = pptr + (offset & (PAGE_CACHE_BITS - 1)) / 32;
 	i = offset % 32;
 	offset &= ~(PAGE_CACHE_BITS - 1);
@@ -74,7 +74,7 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 			}
 			curr++;
 		}
-		kunmap(page);
+		kunmap_local(pptr);
 		offset += PAGE_CACHE_BITS;
 		if (offset >= size)
 			break;
@@ -84,7 +84,7 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 			start = size;
 			goto out;
 		}
-		curr = pptr = kmap(page);
+		curr = pptr = kmap_local_page(page);
 		if ((size ^ offset) / PAGE_CACHE_BITS)
 			end = pptr + PAGE_CACHE_BITS / 32;
 		else
@@ -127,7 +127,7 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 			len -= 32;
 		}
 		set_page_dirty(page);
-		kunmap(page);
+		kunmap_local(pptr);
 		offset += PAGE_CACHE_BITS;
 		page = read_mapping_page(mapping, offset / PAGE_CACHE_BITS,
 					 NULL);
@@ -135,7 +135,7 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 			start = size;
 			goto out;
 		}
-		pptr = kmap(page);
+		pptr = kmap_local_page(page);
 		curr = pptr;
 		end = pptr + PAGE_CACHE_BITS / 32;
 	}
@@ -151,7 +151,7 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 done:
 	*curr = cpu_to_be32(n);
 	set_page_dirty(page);
-	kunmap(page);
+	kunmap_local(pptr);
 	*max = offset + (curr - pptr) * 32 + i - start;
 	sbi->free_blocks -= *max;
 	hfsplus_mark_mdb_dirty(sb);
@@ -185,7 +185,7 @@ int hfsplus_block_free(struct super_block *sb, u32 offset, u32 count)
 	page = read_mapping_page(mapping, pnr, NULL);
 	if (IS_ERR(page))
 		goto kaboom;
-	pptr = kmap(page);
+	pptr = kmap_local_page(page);
 	curr = pptr + (offset & (PAGE_CACHE_BITS - 1)) / 32;
 	end = pptr + PAGE_CACHE_BITS / 32;
 	len = count;
@@ -215,11 +215,11 @@ int hfsplus_block_free(struct super_block *sb, u32 offset, u32 count)
 		if (!count)
 			break;
 		set_page_dirty(page);
-		kunmap(page);
+		kunmap_local(pptr);
 		page = read_mapping_page(mapping, ++pnr, NULL);
 		if (IS_ERR(page))
 			goto kaboom;
-		pptr = kmap(page);
+		pptr = kmap_local_page(page);
 		curr = pptr;
 		end = pptr + PAGE_CACHE_BITS / 32;
 	}
@@ -231,7 +231,7 @@ int hfsplus_block_free(struct super_block *sb, u32 offset, u32 count)
 	}
 out:
 	set_page_dirty(page);
-	kunmap(page);
+	kunmap_local(pptr);
 	sbi->free_blocks += len;
 	hfsplus_mark_mdb_dirty(sb);
 	mutex_unlock(&sbi->alloc_mutex);
-- 
2.37.1